Dermatologists estimate that over 15 million Americans suffer annually from allergic contact dermatitis (ACD) to natural chemicals in plants. Although dermatotoxicological information is available on poison oak and ivy, research on other allergenic plants has been neglected. We propose to continue our research program on the chemistry and dermatotoxicology of allergenic plants in order to better understand the pathogenic mechanism of delayed hypersensitivity reactions caused by contact allergens. We specifically propose to (1) continue the structure elucidation of novel contact allergens and photosensitizers from allergenic plants (2) develop the guinea pig maximization test and the murine animal assay for detecting irritants and immunogens (3) elucidate the mechanism of action of new allergenic quinones, phototoxic lignans and alkaloids (4) develop an enzyme-linked immunosorbent assay (ELISA) for tracing allergenic sesquiterpene lactones which are widespread in the environment and (5) assay potent contact allergens, irritants and photosensitizers for mutagenic and in vitro carcinogenic activity. The knowledge of allergen molecular structure, toxicity and mechanisms of sensitivity will provide clinical dermatologists with basic information for understanding and diagnosing plant-induced allergic contact dermatitis.